1. Field of the Invention
The present invention generally relates to an electronic part mounting substrate and a method for producing the same. More specifically, the invention relates to an electronic part mounting substrate wherein an electronic part, such as a semiconductor chip, is mounted on a metal/ceramic bonding substrate, and a method for producing the same.
2. Description of the Prior Art
In recent years, power modules are used for controlling heavy-current for electric vehicles, electric railcars, machine tools and so forth. In a conventional power module, a metal/ceramic insulating substrate is fixed to one side of a metal plate or compound material called base plate by soldering, and a semiconductor chip is fixed to the metal/ceramic insulating substrate by soldering. On the other side (reverse) of the base plate, a radiating fin or cooling jacket of a metal is mounted via a thermal conduction grease by means of screws.
Since the soldering of the base plate and semiconductor chip to the metal/ceramic insulating substrate is carried out by heating, the base plate is easy to warp due to the difference in coefficient of thermal expansion between bonding members during soldering. Heat generated from the semiconductor chip passes through the metal/ceramic insulating substrate, solder and base plate to be radiated from the radiating fin or cooling jacket to air or cooling water. Therefore, if the base plate warps during soldering, clearance increases when the radiating fin or cooling jacket is mounted on the base plate, so that there is a problem in that heat sink characteristics extremely deteriorate. Moreover, since the thermal conductivity of the solder itself is low, power modules for heavy-current are desired to have more improved heat sink characteristics.
In order to solve such a problem, there is proposed a metal/ceramic circuit board wherein a base plate of aluminum or an aluminum alloy is bonded directly to a ceramic substrate without soldering the base plate to the ceramic substrate (see, e.g., Japanese Patent Laid-Open No. 2002-76551).
In the metal/ceramic circuit board disclosed in Japanese Patent Laid-Open No. 2002-76551, it is not required to solder the base plate to the metal/ceramic insulating substrate, so that thermal conductivity is improved therebetween. However, it remains being required to solder a semiconductor chip to the metal/ceramic circuit board.
In recent years, it is desired to use lead-free solders. Thus, it is also desired to use a lead-free solder when an electronic part, such as a semiconductor chip, is mounted, by soldering, on the metal/ceramic circuit board disclosed in Japanese Patent Laid-Open No. 2002-76551. However, if a high-temperature solder must be used for soldering the part, it is difficult to use a lead-free solder since an alternative lead-free material having characteristics equal to or better than those of conventional high-temperature solders has not been developed.
If an electronic part, such as a semiconductor chip, is fixed by means of a solder, heat sink characteristics are insufficient since the thermal conductivity of the solder is insufficient. In addition, voids are easily produced, so that there are some cases where heat sink characteristics and reliability deteriorate. Moreover, there are some cases where it is required to improve solder wettability by plating a metal plate before the electronic part is mounted on the metal plate by soldering, so that production costs increase.